In marine seismic surveys, a hydrophone cable is towed behind a survey vessel. The hydrophone cable is typically a lead-in cable and a series of seismic streamer sections each formed of an oil-filled plastic tube surrounding an array of hydrophones, strain cables, structural spacers, transformers, and mechanical and electrical leads or connectors. During a survey operation, the seismic streamer sections forming the seismic survey system are towed by the survey vessel at some selected depth below the ocean surface, by any of several means known in the art, such as the use of weighting or paravane structures. For example, 100 feet of hydrophone cable may contain 30 hydrophones whose outputs are combined to form a single hydrophone station signal. For this example, a 12,000-foot cable may contain 120 hydrophone stations. Each station is connected by its own transmission line, or cable pair, to a particular channel of the survey vessel recording instrument. Line lengths of the various hydrophone stations vary considerably. For example, on one survey vessel the cable pair length to the nearest station is 775 feet, while that to the farthest station is 12,695 feet. Each of the hydrophones in the array comprising the hydrophone station is customarily a piezoelectric ceramic capacitance transducer which responds to underwater sound pressure waves and converts such wave phenomena into electrical information, typically an output voltage proportional to the applied acoustic pressure. The cable pair carrying this electrical signal for each hydrophone station is not ideal but has a distributed finite conductivity. In addition, the separate conductors of the cable pair exhibit distributed capacitance between the leads. These characteristics cause the transmitted signal to undergo a frequency selective absorption and phase shift that increases with the length of cable that is traversed. Transformers have been used with varying degrees of success to alleviate the effects of the transmission line by lowering the source or load impedances attached to each cable pair. However, the output voltage on the transmission line is substantially reduced in this method. A loss of sensitivity can result and make some extraneous noise signals more pronounced in their effect.
Another method of reducing the load impedance on a transmission line has been the use of differential charge amplifiers, typified by U.S. Pat. No. 3,939,468 issued to Mastin. In this approach, a differential charge amplifier including two operational amplifiers each having parallel capacitive-resistive feedback loops is coupled to a differential voltage amplifier stage. This circuit has a very low differential input impedance thereby reducing the loss in sensitivity caused by the shunt impedance induced between the single conductors of a cable pair. A high common mode rejection ratio is achieved by using the differential voltage amplifier to cancel the balanced common mode charge response of the differential charge amplifier stage. However, the charge amplifier response in the seismic frequency band matches the output of the hydrophone only for short lengths of transmission line. It does not compensate for series resistance losses which increase with the length of the cable pairs, nor does it compensate for the actual distributed shunt capacitance between the cable pair conductors. Instead, it reduces the effect of total shunt impedance on the transmitted signal by maintaining the two inputs to the charge amplifier stage at approximately the same potential, thereby causing only a low voltage to be induced across the inputs and, hence, very litte current flow through the distributed shunt capacitance or other shunt impedances between the single conductors of the cable pair. Subsequent adjustments must be made to compensate for the reduction in signal amplitude and changes in phase response as the frequency and distance increase.
Since the line lengths among the various hydrophone stations usually differ, the amplitude and phase responses can vary with station number. One method of overcoming this problem involves the addition of passive networks to each cable pair in order to equalize the frequency transfer function for all stations. The design philosophy being used is to make the transfer function for each station approximately the same as that of the station farthest from the survey vessel recording equipment. Once the transfer functions are equalized, subsequent filtering or digital data processing is required in order to complete the compensation so that within a desired frequency passband a flat, zero-phase shift seismic response is obtained from each station.